A tunable laser may be deployed in an optical communications system, such as in a transceiver, a multiplexer, a demultiplexer, and/or the like. The tunable laser may enable use of a flexible wavelength grid for the optical communications system, thereby improving flexibility of the optical communications system relative to fixed wavelength grid implementations. Tunable lasers may include indium phosphide (InP) laser chips, which may be associated with ceramic submounts, that may be deployed in the optical communications system to provide the flexible wavelength grid, and may be dynamically reconfigured to provide different wavelength optical beams during operation. In some cases, a tunable laser may be initially calibrated based on a measurement of an output of the tunable laser and using an active calibration technique. However, an aging drift relationship between electrical drive signals of the tunable laser and an output frequency of the tunable laser may exceed a threshold in optical communications systems, which may cause an initially calibrated tunable laser to lose calibration over time. Thus, a passive calibration technique may be implemented for a tunable laser by using a wavelength monitor.
In this case, the tunable laser may be optically coupled to a wavelength monitor to enable control of the tunable laser. The wavelength monitor enables closed loop frequency control of the tunable laser by providing instantaneous or near-instantaneous measurement of an optical beam being provided by the tunable laser. The wavelength monitor may include one or more calibrated wavelength filters and a set of photo detectors. For example, a first photo detector, of the wavelength monitor, may be a power monitor to measure a tapped portion of an optical beam. Further, a second photo detector, of the wavelength monitor, may be positioned at an output of one or more calibrated wavelength filters, of the wavelength monitor. The second photo detector may provide a measurement of another portion of the optical beam passed through the one or more calibrated wavelength filters.
A ratio of a first optical power measured at the first photo detector to a second optical power measured at the second photo detector may be used to generate a control signal. The control signal may be wavelength dependent and not optical power dependent, which may enable use in calibrating the tunable laser. In other words, an error measurement may be derived from a value of the control signal relative to a target calibrated value for the control signal. However, the wavelength monitor may be associated with a limited wavelength range at which calibration using the control signal is accurately performable. This may be based on sensitivities of the photo detectors in the wavelength monitor.
Thus, a Fabry-Perot type of etalon may be incorporated into the wavelength monitor to increase a sensitivity of the wavelength monitor, which may improve an accuracy of calibration. However, use of a Fabry-Perot type of etalon results in usable wavelength ranges for the wavelength monitor being discontinuous at peaks and valleys of a transmission spectrum of the etalon. The discontinuities may limit use of the wavelength monitor to fixed wavelength grid applications where the tunable laser is to be tuned only to wavelengths within the usable wavelength ranges that occur between the discontinuities.
Accordingly, for flexible wavelength grid applications, another type of wavelength monitor architecture may be used. For example, an optical signal may be separated into three optical paths and directed to three photo detectors. In this case, two optical paths of the wavelength monitor (e.g., waveguides) may include periodic wavelength filters with a common periodicity but a different phase or center frequency, such that the difference is ¼ of the common periodicity. Causing the periodic wavelength filters to differ by ¼ of the common periodicity may enable a relatively high sensitivity and a continuous usable wavelength range for calibrating the tunable laser.
In some cases, the wavelength monitor may be implemented using a set of free space optics (FSO) optical components forming the wavelength monitor. Increasingly, a density of optical components in an optical communications system is being increased, resulting in desirability of miniaturizing the optical components in the optical communications system. Thus, it may be desirable to reduce a form factor and/or a cost of a tunable laser and/or wavelength monitor components associated therewith.